Sand movement in bed-rock channels impacted by dams

Hydropowerdams largely impact sand and gravel movement in rivers. The reservoirs andtheir operation trap the sediment and dampen the flow hydrograph. In fullyalluvial rivers, this causes bed degradation, coarsening and change of channelpatterns. However, in many cases these dams are located in bed-rock channels withunder-supplied conditions. Bed-rock channels are generally steep with highvelocities, and sand is only settling in wakes and eddies behind rocks or alongbanks in sand bars (some sand is stored in pores and pockets in the exposedriver bed as well). Without dams, deposition (during floods) and erosion of thesand bars is balanced, but a dam disturbs this balance and causes gradual degradationof the bars. A well referenced and important example of these conditions is theColorado River in the Grand Canyon in the USA (Grams et al, 2015). The existenceof sand bars is crucial for environmental and social functions of the river. Inthis paper the results of a study to develop a practical approach to simulatethe impacts of hydropower dams on sand bars, and to assess the effects ofmitigation measures. The origin of this study comes from the present plans ofconstructing several dams in the main stream of the Mekong in Laos andCambodia. Previous work of the author for sand-bar modelling in the GrandCanyon using Delft3D (Sloff et al, 2009), showed that with 3D high-resolutionDelft3D models, the sand bar development during artificial floods can be quantifiedto some extent. However, for assessment of large stretches of rivers, suchtechniques require too much computational effort. Instead, a coarser modelling approachhas been developed, that mimics the developments of sand bars and associatedprocesses, but only in an approximate way. The main aim of this approach is toquantify the impacts of modified flows and sediment loads downstream of dams.